Such a transmitter, modulator/demodulator, communication system and method are already known in the art, e.g. from the contribution to the ADSL Standard T1E1.4 with reference number T1E1.4/93-025, written by John M. Cioffi and Po T. Tong, and entitled `VLSI DMT Implementation for ADSL`. Therein, an implementation of a discrete multi tone (DMT) modulator/demodulator dedicated to be used in an Asymmetric Digital Subscriber Line (ADSL) environment is described. The DMT modem comprises a transmitting part and a receiving part, combined in what is called a central office ADSL transceiver on page 1, and a remote terminal ADSL transceiver on page 2. As is seen from FIG. 1 and FIG. 2 of the contribution, these transceivers include a digital signal processor. This digital signal processor (see page 5, paragraph 2.2 and FIG. 4, or see page 12 paragraph 3.2 and FIG. 7) comprises modulation means, named a complex-to-real IFFT (Inverse Fast Fourier Transformer), and demodulation means, named a real-to-complex FFT (Fast Fourier Transformer). As is seen e.g. from FIG. 4, the modulation means IFFT are coupled to a digital to analogue converter DAC, whilst the demodulation means FFT are preceded by an analogue to digital converter, not shown in FIG. 4 but for evident reasons present. The analogue to digital converter in the receiving part and the digital to analogue converter in the transmitting part may operate under control of one clock or different clocks which are synchronised. To cancel within received data symbols, called DMT symbols in the cited document, echoes of transmit data symbols, the transceiver in FIG. 4 further is provided with an echo canceller. If transmit data symbols and received data symbols are not aligned when entering the echo canceller, echo cancellation becomes rather complex since echoes of more than one transmit data symbol should then be cancelled within a received data symbol.
This problem is known e.g. from another contribution to the ADSL Standard, entitled `Modification to DMT Synchronization Pattern Insertion`. This contribution with reference number T1E1.4/93-089 is written by John M. Cioffi and James T. Aslanis. In paragraph 3 thereof, it is recommended to adjust the symbol boundaries of transmitted DMT symbols to coincide with the symbol boundaries of incoming DMT symbols. One of the arguments supporting this recommendation is that it can substantially reduce the complexity of echo cancellers (see page 4, lines 2 and 3).
Misalignment of transmit data symbols and received data symbols occurs when two communicating modems are equipped with independent fixed clocks which control their analogue to digital and digital to analogue converters. Because of the difference between the clock periods of two fixed clocks, the boundaries of transmit data symbols and those of received data symbols will be in relative motion. The problem however not only arises when two communicating modems are equipped with fixed clocks but also exists if these modems make use of controllable clocks, e.g. voltage controlled crystal oscillators, whose clock periods can only be coarse tuned.